A direct-to-home (DTH) satellite television receiving system needs an outdoor satellite receiving antenna and a high-frequency head to receive satellite signals, and the signals are further sent to an indoor satellite television receiver through a cable. In order to achieve good receiving effect, an outdoor satellite receiving antenna is often installed in high places, such as a roof, the top part of a pillar or the open place without obstruction. Then, the satellite receiving antenna becomes an object which is vulnerable to lightning strikes. Once the satellite receiving antenna is struck by lightning, lightning strike overvoltage and lightning current will enter into indoor space through the cable, thereby not only being possible to damage the receiver and other electrical appliances connected therewith, but also being possible to cause fire and personal injuries. The lightning protection becomes an increasingly important problem along with the growing popularization of the DTH satellite television receiving system. For example, the Chinese Patent Publication No. CN2332113Y, entitled “Power supply automatic control device of satellite television ground receiving station” discusses a housing, a circuit board, three working state indicating light-emitting tubes arranged on a panel of the housing, and an input power supply wiring terminal, a power supply output wiring terminal and a manual or automatic selector switch arranged on a backboard of the housing.
The grounding method is traditionally used for lightning prevention, and the antenna body and the cable are grounded before entering into the indoor space, thereby being capable of leading the current into ground when suffering from direct lightning or induced lightning, limiting overvoltage and weakening or eliminating the loss caused by the lightning strikes. As the actual grounding conditions are different in thousands ways, the grounding impedance cannot reach the ideal value; furthermore, due to the presence of multi-point grounding situation, it is still possible to raise the ground potential to the critical value or cause the dangerous potential difference among different grounding points when being struck by lightning. In most cases, the lightning damages cannot be eliminated by only using the grounding measure. The use of a surge protection device (SPD) is another traditional anti-lightning method. In the satellite receiving system, the SPD is often connected between a cable core and a shielding layer in parallel, when the overvoltage appears between the cable core and the grounding external shielding layer, the SPD is conducted for bypassing the surge current into the ground and further limiting the overvoltage between the core and a ground wire. The SPD must be used together with the grounding measure. The SPD can only limit the overvoltage between the core and the ground wire, but not limit the ground potential rise. For example, a detection unit of the Chinese Patent Publication No. CN201015166Y, entitled “Signal anti-lightning type automatic reclosing switch” is connected downstream of the input end of a power circuit in series. A circuit state detection unit is connected upstream of the output end of the circuit. The real-time state of the circuit at the input end and the real-time state of the circuit at the output end are delivered to a processor unit. The switch is further provided with a signal anti-lightning module, and an overcurrent protection device is connected on a signal wire in series and positioned between the input end and the output end.
The above traditional anti-lightning method has certain effects for preventing or reducing the loss caused by the lightning strikes, but cannot completely eliminate the lightning damages. Actually, another effective method is to cut off an intrusion path of the lightning overvoltage and the lightning current, and further take the high-voltage insulation measure between a protected device and an object which may be struck by lightning. For the satellite television receiving system, a cable plug which is connected from the outdoor satellite receiving antenna to the satellite television receiver needs to be pulled out after watching television each time, and then even the antenna is struck by lightning, the receiver and other devices connected therewith cannot be damaged. However, few people do in such a way in practice, this is because it is very inconvenient to pull out the plug after watching television each time and re-plug in the plug before watching television again. Only pulling out the plug before the lightning weather can be more effort-saving, but people need to always pay attention to weather forecasts, thereby being still inconvenient. Furthermore, the plug cannot be pulled out in the event of the lightning weather to avoid the possibility of leading people to be struck by lightning.